The present invention relates to a device and method to hold and immobilize articles and workpieces susceptible to mechanical damage during storage and transport. In particular, the present invention is a containing device and method for storage and transport of solder spheres used in the electronics industries for assembly of printed circuit boards and other electronic assemblies.
Ball grid array (BGA) packages have become the means of choice to house and mount electronic components to larger printed circuit boards and other electronic assemblies. BGA packages typically include a plastic, ceramic or laminated substrate having a specific pattern of pads or recesses on an exposed surface which serve as contact points for mechanically and electrically connecting electronic components, such as integrated circuit chips, to circuit paths of printed circuit boards. BGA packages are mounted to printed circuit boards by surface mount technologies (SMT), including, for example, a process known as reflow soldering. Reflow soldering involves loading solder alloy spheres onto pads or recesses of BGAs and then coupling solder spheres to solder paste applied to specific areas on printed circuit boards which correspond to the array of pads or recesses of BGAs. High temperatures during reflow soldering melt spheres and cause solder to flow around contact points forming mechanical and electrical solder joints between BGAs and printed circuit boards. Accurate placement of solder alloy spheres onto arrays of pads or recesses is, therefore, a critical step in achieving satisfactory mechanical connections and electrical interconnections between integrated circuits housed within BGAs and printed circuit boards.
Automated handling systems, often referred to as pick-and-place systems, are used to load solder alloy spheres to particular soldering sites (pads or recesses) on BGAs. To confirm placement of solder alloy spheres, which can be as small as 0.030 inch in diameter, an automated vision system is often employed to determine if spheres are present at appropriate soldering sites prior to the inception of reflow soldering. An automated vision system rapidly scans an area of loaded solder pads with a high resolution camera. The high resolution camera digitizes the visual data collected to produce a gray level histogram which distinguishes the presence or absence of individual solder spheres by contrasting the bright and shiny surfaces of solder spheres with a darker and more matte background. Such a contrast between the shine of the solder sphere surfaces and the dark background establishes a pass-fail criterion that is used to determine whether a solder sphere is present at a particular site or not. A solder sphere which does not possess sufficient brightness and shine may cause the automated vision system to determine a false fail, indicating that a solder sphere is not present at a particular site, although a solder sphere has, in fact, been accurately placed at the site. The automated vision system may then indicate to an assembling system that a solder sphere is missing, whereby the assembling system may automatically reject the circuit board in question or discontinue production. For such reasons, assemblers and manufacturers of electronic components and systems prefer solder alloy spheres with bright, shiny surfaces in order to avoid erroneous rejection of electronic components and unnecessary production downtime.
It is well known that solder alloy spheres are subject to surface oxidation after manufacture due to an inherent propensity of base metals used in surface alloys, such as tin and lead, to oxidize, causing surfaces of solder spheres to darken. Of particular concern to suppliers of solder spheres is surface darkening produced during storage and transport of solder spheres due to mechanical surface damage or surface fret corrosion which exacerbates oxidation. Surface fret corrosion primarily occurs when surfaces of solder spheres rub against each other causing portions of sphere surfaces to be removed or chipped away. Conventional glass or plastic containers are typically filled with solder spheres by mass, rather than volume, which tends to produce an open or dead space between a bottom planar surface of a container lid and solder spheres contained therein. The dead space enables solder spheres to tumble about the container and to rub against each other, as well as against the side walls of the container, when the container is agitated during storage and transport. Surface fret corrosion is caused by a phenomenon called xe2x80x9cslip-stickxe2x80x9d, whereby mechanical and frictional energy is produced between rubbing sphere surfaces, causing surfaces to bind up rather than slip past each other. Agitation during transport of the container causes bound surfaces to suddenly release, chipping away portions of surface alloy. An accumulated loss of surface alloy by surface fret corrosion causes solder spheres to oxidize and darken.
Prior art packaging and containers which include provisions to prevent mechanical damage to articles contained therein, do not include packaging or containers which immobilize solder alloy spheres. Prior art containers are often designed for specific articles and workpieces. U.S. Pat. No. 5,709,301 discloses a storage container having a foam insert disposed in a container lid to hold and store paint brushes and paint rollers. U.S. Pat. No. 4,347,929 discloses a container for storing and transporting explosives having a foam insert attached to a lid with cut-outs to accommodate blasting caps. In more analogous art, U.S. Pat. No. 4,426,675 discloses a carrier box for storing and transporting printed circuit boards which includes strips of electrically-conductive foam material on the bottom and sides of the container to reduce vibration of printed circuit boards during transport.
Therefore, a container specifically provided for the storage and transport of small solder alloy spheres that includes provisions to prevent movement of spheres, thereby preventing oxidation and preserving the brightness and shine of solder sphere surfaces is desirable.
The present invention relates to a containing device and method to store and transport solder spheres. More particularly, embodiments of the present invention include a containing device to hold and immobilize solder spheres contained therein to prevent mechanical and frictional damage to the surfaces of solder spheres caused by agitation of the containing device during storage and transport.
Embodiments of the present invention include a containing device comprising a hollow container portion having, for example, a circular cylindrical shape with an open top terminal end and an opposing closed bottom terminal end, and a side wall connecting each terminal end thereof. Although a first embodiment of the invention comprises the hollow container portion as a circular cylinder, the geometry of the container portion is not relevant to the invention and, therefore, not limited to any particular shape.
The container portion further comprises a lid which is removably attachable to the open top terminal end to seal the containing device, and a lid insert which is removably insertable into the container portion. The lid insert is of a dimension such that when inserted into the container portion the insert essentially fills an open or dead space between the lid and solder spheres which results once the container device has been filled by mass with solder spheres.
The function of the lid insert is a mechanical one. The lid insert is inserted into the container member by pressure produced from attachment of the lid to the container member. Attachment of the lid compresses the lid insert into the container portion and substantially adjacent to an inner surface of the side wall of the container portion, a bottom planar surface of the lid and an uppermost surface layer of solder spheres. Such firm contact forms an immobilizing seal that prevents movement of solder spheres when the containing device is agitated during storage and transport. The seal also prevents solder spheres from slipping between the lid insert and the side wall of the container portion.
The lid insert may be constructed of suitable compressible materials, such as polymeric foams. Polymeric foams impart a compressible capacity to the lid insert, wherein the lid insert may compress from about 20% to about 50% of its initial volume when inserted into the container portion.
The lid insert may be positioned for insertion into the container portion either manually or with automated handling equipment or similar means. In either case, the attachment of the lid to the container portion, which may be similarly achieved either manually or automatically with handling equipment or similar means, compresses the lid insert into the container portion. The lid insert forms the immobilizing seal as it completely fills the dead space above solder spheres and compresses substantially adjacent to the bottom planar surface of the container lid, the inner surface of the side wall and the uppermost surface layer of solder spheres.
In another embodiment of the invention, the lid insert includes a pull-type tab adhered to a top surface of the lid insert in order to facilitate removal of the lid insert from the container portion when removal of the solder spheres from the containing device is desired.
In another embodiment of the invention, the lid insert includes a semi-rigid or rigid layer of material or a pressure plate disposed on the top surface of the lid insert that is constructed of a thin layer or sheet of flexible, electrostatically dissipative, non-reactive plastic or polymer. The pressure plate imparts rigidity to the lid insert without compromising the compressibility of the lid insert. The pressure plate also provides an optimal surface on which the pull-type tab may be adhered for removal of the lid insert from the container portion.
In still another embodiment of the invention, the pressure plate further comprises a rim disposed along a circumferential perimeter of the pressure plate. The circumferential rim may be constructed of a compressible, non-reactive material including, but not limited to, closed cell polymeric foams and other suitable plastic or synthetic rubber foams, which may or may not be treated for electrostatic dissipative properties.
The circumferential rim compresses and wipes the inner surface of the container portion side wall when the lid insert is compressed into the container portion. The circumferential rim facilitates the formation of the immobilizing seal and prevents solder alloy spheres from penetrating the seal and becoming lodged between the inner surface of the side wall and the lid insert.
In yet another embodiment of the invention, the lid insert is adhered to the bottom planar surface of the lid such that when the lid is attached or detached from the container portion, the foam insert is inserted or removed from the container portion.